The decomposition of water by the action of solar energy is known. This decomposition is based on the formation, in an aqueous solution, by radiation with visible light, of activated complexes of a redox system capable of reacting on suitable catalysts in order to yield H.sub.2 and O.sub.2.
Thus, for instance, there is known the photooxidation of water by means of Fe.sup.3+ and quinone in the presence of ZnO, TiO.sub.2 and WO.sub.3.
Another reaction, used for photo-chemically decomposing water, foresees the use, in the presence of Pt and RuO.sub.2 colloidal catalysts, of a redox system formed of ruthenium trisbipyridyl and methylviologen (N,N'-dimethylbipyridine).
More precisely, the system works in the following way: the ruthenium trisbipyridyl, in its reduced form Ru(bipi).sub.3.sup.2+, reduces the methylviologen MV.sup.2+ under the action of visible light, thereby obtaining:
(1st) the reduced MV.sup.+ which reduces the water to H.sub.2 on catalyst Pt, regenerating MV.sup.2+ ; PA1 (2nd) the oxidized Ru(Bipi).sub.3.sup.3+ which oxidizes the water to O.sub.2 on catalyst RuO.sub.2, thereby regenerating Ru(bipi).sub.3.sup.2+.
The yields in H.sub.2 and O.sub.2, obtained through this method, are very low.
Furthermore, it is known to use a ruthenium trisbipyridyl-methylviologen redox system in the presence of a catalyst of colloidal platinum and EDTA (ethyldiaminetetraacetic acid).
The EDTA is oxidized by the oxidized form of ruthenium trisbipyridyl, wherefore only H.sub.2 is formed, with a yield that is greater than that obtained using the redox system formed of ruthenium trisbipyridyl and methylviologen in the presence of Pt and RuO.sub.2 colloidal catalysts, but still rather low.